1. Field of the Invention
The present invention relates to a hard coat film for optical applications and, more particularly, a hard coat film for optical applications which suppresses formation of defects on the surface derived from roughness of the surface of the substrate film, exhibits excellent scratch resistance and crack resistance, suppresses curling of the substrate film, has the function of a hard coat and is advantageously used for a film for a touch panel, a polarizing plate, a ¼ wavelength plate, a cover film of optical disks and a protective film for various displays.
2. Description of Related Art
Touch panels are used as the input apparatus for portable information terminals which are being widely used in recent years. The touch panel is an apparatus for inputting data by directly touching the surface of a display by a finger or a pen.
About 90 percent of the entire touch panels are the touch panels of the resistance film type. The touch panel of the resistance film type has, in general, a structure in which a plastic substrate of the touching side which has a transparent electrically conductive (referred to as “conductive”, hereinafter) thin film such as a film of indium oxide doped with tin (ITO) laminated on one face of a transparent plastic substrate and a transparent substrate of the display side which has a transparent conductive thin film such as an ITO film laminated on one face of a transparent substrate material such as a glass plate are arranged with an insulating spacer placed between the two substrates in a manner such that the transparent conductive thin films on the two substrates are faced to each other.
For the input operation, the face for input by touching (the face opposite to the face having the transparent conductive thin film) of the plastic substrate of the touching side is pressed by a pen or a finger and the transparent conductive thin film of the plastic substrate of the touching side and the transparent conductive thin film of the display side are brought into contact with each other.
However, the touch panel of the resistance film type has a problem in that the transparent conductive thin film on the plastic substrate of the touching side is worn, has cracks or is separated from the substrate after repeated input operations, i.e., after the transparent conductive thin film of the plastic substrate of the touching side and the transparent conductive film of the transparent substrate of the display side are repeatedly brought into contact with each other. To overcome the above problem, it is widely conducted that a hard coat layer of a cured resin is disposed between the transparent plastic substrate and the transparent conductive thin film. It is also widely conducted that a hard coat layer is disposed on the surface of the transparent plastic substrate opposite to the surface on which the transparent conductive thin film is disposed.
Development of optical disks as the information recording media in recent years is remarkable. Already, disks used only for reading information such as CD and CD-ROM, optomagnetic disks called MOD or MD, phase-transition disks called PD and writable optical disks called CD-R are widely used. The recording capacity of these disks is about 650 MB (megabytes). Disks having greater capacities such as a series of optical disks called DVD (digital versatile disk) are developed. Specific examples of DVD include DVD-ROM (DVD allowing reading only), DVD-R (DVD allowing additional writing), DVD-RAM (DVD allowing writing and reading) and DVD-RW (DVD allowing rewriting). In these optical disks, a hard coat film is frequently used as the protective film and the cover film.
Various instruments having a liquid crystal display apparatus which are, for example, consumer appliances such as laptop computers, video cameras and portable telephones, displays for instrument panels of automobiles and airplanes and liquid crystal projectors, are being widely used in recent years, and colored display and improvement in luminance and durability are required.
The liquid crystal display apparatus is an apparatus which modulates the incident linearly polarized light using the electro-optical property of the liquid crystal layer and visualizes an image by the difference in the magnitude of transmittance or by color signals using a polarizing plate at the side of emission of the light. In other words, polarized light is used in the principle of displaying by the liquid crystal display apparatus, and the polarizing plate is the essential member of the apparatus. The polarizing plate is a device for changing natural light into a linearly polarized light. In many of the polarizing plate currently mass produced and practically used, in particular, for liquid crystal display apparatuses, a plate prepared by laminating an optically transparent protective film having mechanical strength to one or both faces of a substrate film which is a polarizing film made of polyvinyl alcohol film, tinted or impregnated with iodine or a dichroic material such as a dichroic dye and stretched for orientation, is used. As the protective film, in general, films of triacetylcellulose are used.
In the liquid crystal display apparatus, in general, the polarizing plate described above is disposed not only at the side of emitted light but also at the side of incident light. When the polarizing plate is disposed on the surface side of various display apparatuses such as the liquid crystal display apparatus, sufficient scratch resistance and hardness are required, and it is preferable that the plate has the properties of preventing fouling.
In the liquid crystal display apparatus, in general, the light emitted from the light source is circularly polarized by a circularly polarizing member, then linearly polarized through a ¼ wavelength plate and supplied to the polarizing plate. A hard coat film is disposed also on the ¼ wavelength plate.
When a display such as a plasma display panel (PDP), a cathode ray tube (CRT), a liquid crystal display apparatus (LCD) and an electroluminescence device (ELD) is used, light from the outside is occasionally reflected at the surface of the display and difficulty arises in watching images on the display. In particular, as the size of flat panel displays increases recently, solving the above problem is becoming more important.
To solve the above problem, various treatments for preventing reflection of light and anti-glare treatments have been made on various display apparatuses. As one of such treatments, a film for preventing reflection of light is used for various display apparatuses. For the film for preventing reflection, the hard coat property, i.e., scratch resistance of the surface, is required in combination with the property of preventing reflection.
For preparation of the film for preventing reflection of light, when a dry process such as vapor deposition and sputtering is used, a thin film of a substance having a low refractive index such as MgF2 is formed on a substrate film, or layers of a substance having a great refractive index such as ITO and TiO2 and layers of a substance having a small refractive index such as MgF2 and SiO2 are alternately laminated. When a wet process is used, a hard coat film having the property of preventing reflection of light is prepared in accordance with the coating process. In the wet process, for example, a film of an acrylic resin exhibiting excellent weatherability is used as the substrate. After a layer of a cured resin composition sensitive to an ionizing radiation is formed, the film is treated for preventing reflection of light and used as the protective film for the liquid crystal display apparatus of portable telephones, portable information terminals (PDA) and video cameras.
It is also attempted in a liquid crystal display apparatus that a film for preventing reflection of light is disposed on the protective film of the polarizing plate described above.
For the hard coat film for optical applications used as a touch panel, a polarizing plate, a ¼ wavelength plate, a cover film for optical disks or a protective film of the surface of various displays, it is required that the surface have a great hardness and excellent scratch resistance, form little interference fringes, suppress curling of the film and have few defects derived from roughness of the surface of the substrate film so that excellent clarity of vision through and operability are surely exhibited.
Heretofore, in the preparation of the hard coat film for optical applications, the thickness of the hard coat layer is adjusted so that the above requirements are satisfied. In the actual practice, it is necessary that a hard coat layer having a thickness greater than the really necessary thickness be formed to decrease defects on the surface derived from the roughness of the surface of the substrate film. Under this situation, curling of the hard coat film increases due to shrinkage in the volume of the hard coating agent, and problems arise in that the property for handling deteriorates and that cracks are formed. To decrease the curling of the hard coat film, it is proposed that the surface of the base film (the substrate film) is irradiated with ultraviolet light (for example, Japanese Patent Application Laid-Open No. 2001-205179). However, this method has problems in that the effect of suppressing the curling is insufficient and that the amount and the time of irradiation with ultraviolet light must be increased to sufficiently cure the hard coat layer.